1. Field of the Invention
The present invention relates to a liquid ejection head, a substrate for a liquid ejection head, a liquid ejecting apparatus including a liquid ejection head, and a method of cleaning a liquid ejection head.
2. Description of the Related Art
Recording methods using liquid ejection are methods of performing recording by ejecting a liquid (e.g., ink) from ejection ports provided in a liquid ejection head and allowing the liquid to adhere to a recording material such as paper. Among these recording methods, a liquid-ejection recording method in which a liquid is ejected by utilizing bubbling of the liquid formed by thermal energy generated by an electrothermal transducer can realize a high image quality and a high-speed recording.
A liquid ejection head typically includes a plurality of ejection ports, a flow passage communicating with the ejection ports, and a plurality of electrothermal transducers that generate thermal energy used for ejecting ink. Each of the electrothermal transducers includes a heating resistor layer, an electrode configured to supply the heating resistor layer with an electric power, and an insulating lower protective layer composed of, for example, silicon nitride and covering the heating resistor layer and the electrode. Thus, insulation is ensured between the ink and the electrothermal transducer.
A heating portion used as the electrothermal transducer during liquid ejection is exposed at high temperatures and undergoes a cavitation impact due to bubbling and contraction of a liquid and a chemical action due to ink in various manners. Therefore, in order to protect the heating resistor layer from such a cavitation impact and a chemical action due to the ink, an upper protective layer is provided on the heating portion. The temperature of a surface of the upper protective layer increases to about 700° C., and the surface contacts the ink. Accordingly, it is necessary that the upper protective layer have good film characteristics in terms of heat resistance, mechanical properties, chemical stability, alkali resistance, etc.
Furthermore, a coloring material, additives, and the like contained in the ink may be decomposed on the molecular level by heating at a high temperature and changed to a substance called “kogation”, which is not readily dissolved. When such kogation is physically adsorbed on the upper protective layer, heat conduction from a heating resistor to the ink becomes nonuniform and thus formation of bubbles becomes unstable.
To solve this problem, US 2007/0146428 discloses a technique for removing kogation by dissolving a surface of an upper protective layer composed of iridium or ruthenium by an electrochemical reaction.
In the technique described in US 2007/0146428, the amount of reduction in the thickness of the upper protective layer due to the dissolution by the electrochemical reaction depends on the concentration of an electrolyte contained in ink used in the electrochemical reaction. Accordingly, it is a matter of concern that the amount of reduction in the thickness of the upper protective layer becomes variable because of a variation in the concentration of an electrolyte contained in ink or a variation in the type of ink. Such an uneven thickness of the upper protective layer in a head may degrade the recording quality. Accordingly, in a head in which a plurality types of ink having different colors are used, it is necessary to set conditions for an electrochemical reaction for each color. Furthermore, in some cases, the amount of dissolution of the upper protective layer may be larger than an assumed amount, and thus the electrochemical reaction cannot be conducted a predetermined number of times.